1. Field of the Invention
The present invention relates to a transmitter suitable for a CDMA (Code Division Multiple Access) mobile terminal.
2. Description of the Related Art
In a mobile communication with a spread spectrum scheme (hereinafter referred to as a CDMA scheme), a plurality of mobile terminals share the same frequency band with one another and each of the mobile terminals spreads transmission data with a spreading code and transmits the spread data to a base station. The mobile terminals are respectively assigned different spreading codes each having orthogonality. The base station can extract data received from a particular mobile terminal by performing despread with the same spreading code as that used by the mobile station for spread. A particular mobile terminal considers all the transmission power of the other mobile stations which share the same frequency band with the mobile terminal as noise.
For this reason, in the mobile communication with the CDMA scheme, control (hereinafter referred to as transmission power control) for suppressing the transmission power of all mobile terminals to a minimum level required for ensuring a predetermined error rate is performed for reducing a noise level to other mobile terminals to provide as many mobile terminals as possible which can share the same frequency band with one another, thereby achieving effective utilization of radio frequencies and a reduction in equipment cost of a system or the like. Therefore, exact transmission power control is required for a mobile communication system with the CDMA scheme.
In addition, in the CDMA scheme, since a signal with a spread wide frequency bandwidth is transmitted, a transmitter with a wide dynamic range is required in the transmission power control. In particular, a transmitter of a mobile terminal needs to realize a small size, light weight, and low power consumption.
In the transmitter, however, a phenomenon called carrier leak occurs in which a carrier for use in modulation directly appears on an output from a modulator. When an output from a transmitter is high, the effect of the carrier leak causes no problem since the level of the carrier leak is negligible as compared with the level of an output signal from the modulator. However, when the output from the transmitter is low, the effect of the carrier leak is not negligible.
Conventionally, circuits for suppressing the carrier leak include a circuit as described in Japanese Patent Laid-open Publication No. 8-204772 in which the phase of a carrier signal is reversed by 180 degrees and combined with an output from a modulator, or a feedback circuit as described in Japanese Patent No. 2852294 in which an output level from a modulator is monitored and a carrier output is suppressed when the level is low.
In the circuit described in Japanese Patent Laid-open Publication No. 8-204772, it is necessary to adjust the phase and level of a signal to be combined while a spectrum analyzer makes observations, which renders the circuit unsuitable for use in a mass-produced mobile terminal. In the circuit described in Japanese Patent No. 2852294, since an output from a modulator needs to be split with a directional coupler toward a detection circuit, the circuit is not optimal for a mobile terminal which requires a small size, light weight, and low consumption power in terms of attenuation due to loss in the directional coupler and the circuit scale. In addition, since the output level of the modulator is monitored to control the carrier output, the control of the carrier may be delayed if transmission power is suddenly changed.
It is an object of the present invention to provide a transmitter for a mobile terminal of small size, light weight, and low consumption power including a carrier leak suppressing circuit.
It is another object of the present invention to provide a transmitter for a mobile communication terminal in which characteristics of a carrier leak suppressing circuit can be optimally changed after the start of operation.
According to one aspect of the present invention, the transmitter has a carrier control circuit including reference voltage generating means, a comparator, and a controller as a carrier leak suppressing circuit.
The comparator compares the level of a powder control signal with a reference voltage. The controller controls the gain of a local variable gain amplifier in a carrier generating circuit such that a distortion characteristic of a transmitter output is optimal when the power control signal is higher than the reference voltage and the effect of carrier leak on the transmitter output is suppressed when the power control signal is lower than the reference voltage based on the comparison result of the comparator.
Therefore, the use of the carrier control circuit in a transmitter of a mobile terminal which exerts a changeable output level with transmission power control can provide a transmitter output such that distortion is optimal when the transmitter output is high, or the effect of carrier leak is suppressed when the transmitter output is low, thereby allowing much channel capacity to be ensured for a mobile communication system. In addition, the use of the control signal for the control of a carrier eliminates the need for splitting and detecting the output from a modulator to enable realization of a small size and low consumption power. Moreover, since the carrier control and transmission power control are simultaneously performed using the control signal, the carrier control is not delayed with respect to the transmission power control, resulting in the suppression of carrier leak with appropriate timing.
According to an embodiment of the present invention, the reference voltage generating means generates a reference voltage equal to a level of a control signal corresponding to transmission power equal to the level of carrier leak.
Therefore, the level of carrier leak is not higher than the output from the modulator.
According to another embodiment of the present invention, the comparator has a hysteresis characteristic in the determination of whether the level of the power control signal is higher or lower than the reference voltage.
Therefore, instability of control can be prevented when the voltage level of the power control signal is increased and reduced near the reference voltage, thereby obtaining a stable output from the modulator.
According to another embodiment of the present invention, the reference voltage is changeable. This facilitates initial settings, adjustment for adapting the reference value to variations in characteristics of circuit parts or the like, and enables changes in a reference code value or a hysteresis width after the start of operation. Thus, manufacturing processes can be reduced and optimization after the start of operation can be performed.
According to another aspect of the present invention, the carrier control circuit includes a reference value register, a comparator, and a controller.
The comparator determines whether a control code is higher or lower than a predetermined reference code which is the output from the reference value register. The controller controls the gain of the local variable amplifier such that a distortion characteristic of a transmitter output is optimal when the control code is higher than the reference code, and controls the gain of the local variable amplifier such that the effect of carrier leak on the transmitter output is suppressed when the control code is lower than the reference.
Therefore, the use of the carrier control circuit of this configuration in a transmitter of a mobile terminal which exerts a changeable output level with transmission power control can provide a transmitter output such that distortion is optimal when the transmitter output is high, or the effect of carrier leak is suppressed when the transmitter output is low, thereby allowing much channel capacity to be ensured for a mobile communication system. In addition, the use of the control code for the control of a carrier eliminates the need for splitting and detecting the output from the modulator to enable realization of a small size and low consumption power. Moreover, since the carrier control and transmission power control are simultaneously performed using the control code, the carrier control is not delayed with respect to the transmission power control, resulting in the suppression of carrier leak with appropriate timing.
According to an embodiment of the present invention, the reference value register outputs a reference code equal to a control code corresponding to transmission power equal to the level of carrier leak. Thus, the level of carrier leak is not higher than the output from the modulator.
According to another embodiment of the present invention, the comparator has a hysteresis characteristic in the determination of whether the value of the power control code is higher or lower than the reference code. Therefore, instability of control can be prevented when the value of the control code is slightly increased and reduced repeatedly near the reference code, thereby obtaining a stable output from the modulator.
According to another embodiment of the present invention, the reference code generated by the reference value register is changeable. This facilitates initial settings, adjustment for adapting the reference value to variations in characteristics of circuit parts or the like, and enables changes in a reference code value or a hysteresis width after the start of operation. Thus, manufacturing steps can be reduced and optimization after the start of operation can be performed.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.